Image forming apparatuses such as an electrographic printing machine, an electrographic printer, an electrographic facsimile, etc. have been conventionally known. In an electrographic image forming apparatus, a toner image is formed on a surface of a photoreceptor by forming an electrostatic latent image on the surface of the photoreceptor and developing it by use of a toner supplied from a developing apparatus to the surface of the photoreceptor. Then, the toner image is transferred to a sheet such as a paper, etc., and fixed on it by a fixing apparatus.
The electrographic image forming apparatus has a problem that an image density is changed depending on a change in an environment condition around the electrographic image forming apparatus, weariness of the photoreceptor and/or a developer, etc.
In view, the image forming apparatus is arranged such that process control for adjusting the image density is performed for every predetermined numbers of sheets of printing or in accordance with an environment condition such as a temperature, a humidity, etc.
According to the process control, first, a plurality of small toner images (hereinafter referred to as toner patches) having sequentially varied densities are formed on a surface of a photoreceptor (see Patent Literature 1, for example). Then, the densities of the toner patches are detected by a density sensor. After this, control for adjusting an image density is carried out based on a result of the detection. Specifically, a developing bias (hereinafter referred to as a reference developing bias), which serves as a reference for a targeted image density (hereinafter referred to as a target image density), is determined based on the densities of the respective toner patches thus detected by the density sensor.
The density sensor detects the densities of the toner patches that are formed on the surface of the photoreceptor, or, in an arrangement including an intermediate transfer belt, the density sensor detects densities of toner patches that are transferred onto a transfer belt from a surface of a photoreceptor.